Accurately characterizing such materials and structures can be a challenge. Standard measurement techniques, usually based on strain gauges, provide only limited and local information. The rapid development of digital camera technology in combination with high performance digital image correlation (DIC) techniques is currently bringing about change in this field.
Thanks to DIC, it is now possible to extract full field 3D geometry, displacement, strain and acceleration, under any load and for almost any type of material, with limited instrumentation. The technique is useful for measuring the strain of materials and structures under static and quasi-static loading. The DIC is also applicable to dynamic measurements and has recently been extended to deal with vibration measurements and modal testing of structures.
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The principle of digital image correlation is simple but powerful. This optical-digital measurement technique extracts full-field information about the object under test from images captured with digital cameras. The DIC algorithm will compare all images with a reference and extract the shape and displacement amounts in the region of interest over thousands of measurement points and with sub-pixel precision.
Once the displacements are available, the strains, stresses and other derived kinematic quantities can be immediately derived. As a result, it is possible to get a detailed view of the entire strain field of the object under test as the loads change, monitor the strain level and identify local regions with critical stress levels.
Simcenter Testlab offers comprehensive, modular software for efficient image acquisition and provides quantitative and accurate full-field results based on the DIC technique. As the industry moves more and more towards virtual-based product design, it is obviously essential to have accurate information about the real properties of materials.
Thanks to a large library of material models and the implementation of the virtual field method, the solution makes it possible to take advantage of full-field experimental results and extract precise material parameters directly from the measured data. DIC can also be used to analyze finite element (FE) model results and to ensure a reliable and quantitative comparison between FE and full field experimental data.
DIC is also a complementary technique to traditional approaches to vibration measurements based on discrete accelerometers. It relies on very limited instrumentation and full field information can provide more information on the dynamic behavior of aerospace structures.
Once the DIC data is processed, it can then be used for modal or operational analysis. The DIC can be used for vibration testing as a stand-alone system or in combination with other vibration sensors. The latter scenario proposes, for example, to derive frequency response functions from measured travel time histories, or to combine the overall structural behavior captured with accelerometers with local full-field information from a DIC system.
The technique allows a wide range of structures to be tackled from small samples of materials or components to very large structures such as integrated airplanes for ground vibration testing. DIC does not necessarily require high speed cameras to measure vibrations and characterize the modal behavior of aerospace structures. More affordable low speed cameras can also be used. In this case, a resampling technique is automatically applied after the measurement which extracts vibration data at frequencies well above the frame rate limit of the cameras.
DIC offers new capabilities to effectively validate complex materials and aerospace structures. Full-field information obtained with limited instrumentation are key elements that make it highly complementary to traditional measurement techniques.
DIC can provide full field vibration information on complex aerospace structures
This article has been provided by Siemens